Industrial production plants often generate hot or flue gases. Such flue gases must usually be cooled for proper operation of the production plant. In these applications, the flue gases are often passed through various portions of the production plant to provide a cooling effect. In other cases, however, additional cooling and conditioning systems must be utilized to produce the proper temperature. The flue gas is sometimes cooled by injecting an atomized liquid stream into the gas stream, such as through spraying water with very fine droplets into the gas stream. This operates to reduce the temperature of the gas stream.
There are typically various cooling requirements for a production plant of the general type described above. For example, the outlet temperature is typically required to be maintained at a particular temperature level or temperature set-point. Inasmuch as the flue gases typically raise the outlet temperature above the set-point value, the system is required to reduce the outlet temperature. In addition, complete evaporation of water contained within the exiting gas must be accomplished within a given distance (dwell distance). That is, all or substantially all of the liquid is required to be evaporated within a given distance of the location of the spray nozzle or nozzles to avoid undue wetting of the various components of the system. These usually include a filtration system, e.g., bag-house and other components.
For providing a liquid spray, such systems sometimes employ one or more bi-fluid nozzles. The nozzles use compressed air as an energy carrier to atomize a liquid, such as water, into fine droplets. In most systems today, the air pressure used for spray nozzles of this type is kept constant over the operating cooling range. The amount of constant air pressure required is usually calculated based on the maximum allowed droplet size for obtaining total evaporation, a parameter known to those skilled in the are as Dmax (i.e., maximum droplet size), within a given distance at the worst cooling conditions (usually at maximum inlet gas temperature and maximum inlet gas flow rate).
Of course, less liquid spray is required to cool the gas to the desired temperature when the inlet gas flow rate or inlet temperature decreases. Maintenance of a constant air pressure in these circumstances causes the air-flow rate to increase. This results in increased air consumption and in increased compressed air energy cost. For maintaining the cooling requirements of the system, it is often unnecessary to maintain the air pressure constant at lower cooling conditions. Thus, it would be desirable to closely monitor these parameters of the system to enable appropriate adjustment of air pressure provided to the atomizing spray nozzles as necessary or desired.